Lances intended for metallurgical processes consist in principle of a steel tube provided with a nozzle at one end and a lance holder at the order, the tube being surrounded by a refractory casing or sheath. This refractory sheath was originally made up from annularly shaped refractory bricks, joined to each other with refractory mortar. In this case, however, as with most metallurgical sheaths, there is a tendency to go over to a so-called monolithic sheath, which is generally more resistant and more simple to produce.
Monolithic sheaths have the disadvantage of not being self-supporting to the same extent as those made from prefabricated bricks. They have not been able to be made with an insulating air gap between the sheath and the lance pipe, as with the sheaths made from annularly shaped bricks, since attempts to do so have much too often led to cracks in the sheath, with the risk of molten metal penetrating through the cracks to the lance pipe and melting it. The monolithic sheaths have so far usually been made by moulding a refractory composition in a mould in which the lance pipe forms the inner wall. Anchoring pieces have usually been welded onto the lance pipe to ensure retention of the sheath. However, such a construction also leads to the formation of cracks in the sheath due to thermal stresses caused by the great temperature difference between the sheath which is in contact with the molten metal having a temperature of say 1600.degree. C., and the lance pipe, which can have a temperature below 100.degree. C. due to the heavy air blast through it.